Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various material layers using lithography to form circuit components and elements thereon.
An image sensor is a semiconductor device for converting an optical image into an electrical signal. The image sensor is roughly classified as a charge coupled device (CCD) or a CMOS image sensor (CIS). One type of the CIS is Front Side Illumination (FSI) image sensor. In the FSI image sensors, light travels to a photo-sensing area through the front side of the pixel. This means that the incident light has to first pass through dielectric layers, and metal layers before it strikes the photo-sensing area, causing low quantum efficiency (QE), serious cross talk between pixels, and dark current. Another type of CIS is Back Side illumination (BSI) image sensors. Instead of illuminating a CMOS image sensor from the top (front) side of the silicon die, a BSI image sensor applies the color filters and the microlens to the back side of the pixels so that the incident light is collected from the back side of the image sensor. Compared to the FSI image sensors, the BSI image sensors have less light loss, reduced crosstalk, and better quantum efficiency.
Although existing image sensors have been generally adequate for their intended purpose, they have not been entirely satisfactory in all aspects.